The present invention relates to the preparation of ethylene dichloride, and in particular to the addition chlorination of ethylene. In its preferred form, the invention relates to a novel process for preparing 1,2-dichloroethane by addition chlorination, wherein the heat generated by the reaction of chlorine and ethylene is used to vaporize and rectify the ethylene dichloride produced, and is also used to vaporize and rectify crude ethylene dichloride from other sources.
The preparation of dichloroalkanes by the chlorination of an appropriate olefin in the liquid phase under suitable conditions is well known. British Pat. No. 1,231,127 discloses a method of preparing dichloroalkanes wherein an olefin is addition chlorinated (with chlorine) in the liquid phase at suitable temperatures, whereby the dichloroalkane formed is distilled or vaporized, and is passed to a closely related distillation column which is also fed with crude dichloroalkane obtained from at least one other source. The heat of reaction contained in the vaporized dichloroalkane is used to rectify the dichloroalkane vapor, and because of the large excess of heat present, is used additionally to rectify the crude dichloroalkane from another source, e.g., crude dichloroalkane obtained from the oxychlorination of an olefin, and/or unconverted recycle dichloroalkane from a pyrolysis system in which dichloroalkane is pyrolyzed to a given monochloroalkene.
This procedure has a number of disadvantages. To begin with, severe problems arise from the introduction of an ethylene oxychlorination effluent directly into the fractionation column, as described in the patent. For example, water present in oxychlorination effluents, unless removed prior to fractionation, will provide severe corrosion and separation difficulties in the fractionation zone, its condenser, and the chlorinator. Moreover, the crude ethylene dichloride obtained from the oxychlorination of ethylene, even after preliminary purification, often contains significant amounts of trichloroethylene. This trichloroethylene cannot be removed easily by fractional distillation, and may act as a reaction inhibitor when the ethylene dichloride product is later dehydrochlorinated. The oxychlorination effluent contains minor amounts of chloral, i.e., trichloroethanol, which pose additional separation problems. Again, the residual ethylene dichloride supplied to the distillation column from the pyrolysis system usually contains small amounts of chloroprene, i.e., 2-chloro-1,3-butadiene. Unless eliminated prior to entry into the fractionation zone, the chloroprene tends to polymerize and foul the column and condenser.